Exhaust gas cleaner system for an internal combustion engine with catalytic converter supplied with secondary air

ABSTRACT

Exhaust gas cleaner system for an automotive internal combustion engine includes an air introduction pipe 8 for introducing secondary air, heated initially by a heater 23, to the catalytic converter unit 7 mounted on the exhaust pipe 3. The ignition timings of the engine is retarded at least during the time when the secondary air is heated by the heater 23. Preferably, the number of ignition timings are counted by a counter modulo 5, for example, in the case of a four-cylinder engine, and the ignition timings are retarded when the counter content is 3 or 4, such that the ignition timings of the cylinders are retarded intermittently. Alternatively, the retardation of the ignition timings may be terminated before the introduction of the heated secondary air is terminated. 
     Where two first and second catalytic converter units 7a and 7b are provided, secondary air distributed by a change-over valve 10a is heated separately by heaters 23b and 23c. 
     To reduce the burden imposed on the engine, the generation of power by the AC generator 28 may be suspended when either the flow control valve 10b or the heater 23 is operated.

This is a divisional of application Ser. No. 08/269,681 filed Jul. 1,1994, now U.S. Pat. No. 5,459,999.

BACKGROUND OF THE INVENTION

This invention relates to exhaust gas cleaner systems includingcatalytic converters for cleaning the exhaust gas of the internalcombustion engines, and more particularly to the secondary airintroduction systems for introducing secondary air into the exhaust pipeof the engine for enhancing the cleaning efficiency of the catalyticconverters.

Catalytic converters are provided on the exhaust pipe of the automotiveinternal combustion engines for cleaning the exhaust gas. When heatedabove the reaction temperature, the catalytic converters remove thenoxious components efficiently from the exhaust gas. Immediately afterthe start of the engine, however, the temperature of the catalyticconverter is still low and hence the cleaning efficiency thereof isinsufficient. To enhance the cleaning efficiency, fresh air (secondaryair) is introduced into the exhaust pipe upstream of the catalyticconverter. The air promotes the oxidation of noxious components such asHC (hydrogen carbide) and CO (carbon monoxide), and thereby raises thetemperature of the catalytic converter quickly and enhances the cleaningefficiency (the efficiency to remove the noxious components) thereof.

FIG. 1 is a block diagram showing an internal combustion engine providedwith a catalytic converter supplied with secondary air through an airintroduction pipe. To an automotive internal combustion engine 1 areconnected an air intake pipe 2 for supplying air to the engine and anexhaust pipe 3 for exhausting the exhaust gas to the atmosphere. Theexhaust gas contains noxious components generated by the combustionwithin the cylinders of the internal combustion engine 1. The noxiouscomponents include nitrogen oxides (NO_(x)) which are produced inabundance at higher temperatures and should be reduced by the catalyticconverter for removal, and HC (hydrogen carbide) and CO (carbonmonoxide), which are produced in abundance at lower temperatures andshould be oxidized by the catalytic converter for removal.

At an upstream point of the air intake pipe 2 is disposed an air cleaner4 for removing dusts contained in the air supplied to the cylinders ofthe engine. To the downstream side of the air cleaner 4 is mounted anairflow sensor 5 for measuring the flow rate of air taken into thecylinders of the internal combustion engine 1. Further downstream of theairflow sensor 5 is disposed a throttle valve 6 for adjusting the amountof airflow supplied to the cylinders of the engine.

At a middle point of the exhaust pipe 3 is disposed a catalyticconverter unit 7 accommodating the catalytic converter (e.g., catalyticconverter rhodium) for cleaning the exhaust gas by means of chemicalreactions. An air introduction pipe 8 is coupled across a point of theair intake pipe 2 downstream of the air cleaner 4 and a point of theexhaust pipe 3 upstream of the catalytic converter unit 7. An air pump 9forces the air from the air intake pipe 2 into the exhaust pipe 3, andintroduces the secondary air into the exhaust pipe 3. A check-valve 10prevents the exhaust gas from reversing through the air introductionpipe 8 into the air intake pipe 2.

An air/fuel ratio sensor 11 on the exhaust pipe 3 detects the oxygenconcentration of the exhaust gas. Fuel injectors 12 are provided forrespective cylinders of the engine in the respective branches of the airintake pipe 2. From the output of a crank angle sensor 13 is detectedthe rpm of the engine. The outputs of the airflow sensor 5, the air/fuelratio sensor 11, and the crank angle sensor 13 are supplied to an enginecontroller 14, which controls the operations of the fuel injectors 12 inresponse to the outputs of the sensors 5, 11 and 13. The ignition plugs(not shown) for the respective cylinders are supplied from an ignitioncoil 15. The current supply to the ignition coil 15 is controlled by anigniter 16, which is controlled by the engine controller 14.

The operation of the secondary air introduction system for the catalyticconverter 7 of FIG. 1 is as follows. A part of the air passing throughthe air cleaner 4 is sucked into the air introduction pipe 8 by means ofthe air pump 9 and introduced, through the check-valve 10, into theexhaust pipe 3 at a point upstream of the catalytic converter unit 7.The secondary air thus introduced into the exhaust pipe 3 is mixed withthe exhaust gas exhausted from the cylinders of the engine and then ispassed together through the catalytic converter unit 7. The secondaryair supplied through the air introduction pipe 8 promotes the oxidationof the noxious components such as HC and CO into corresponding innocuouscompounds such as H₂ O (water) and CO₂ (carbon dioxide). The exhaust gaspassed through the catalytic converter is released into the atmosphere.

The fuel injectors 12 are controlled by the engine controller 14. Theamount of the injected fuel is determined primarily on the basis of theoutput of the airflow sensor 5 and the rpm of the engine calculated fromthe output of the crank angle sensor 13. The amount of the injected fuelthus determined is corrected on the basis of the output of the air/fuelratio sensor 11, etc., and the fuel injectors 12 are driven inaccordance with the corrected amount.

The igniter 16 controls the ignition timings of the respective cylindersby turning on and off the current supplied to the ignition coil 15. Theengine controller 14 controls the operation of the igniter 16 inresponse to the output of the airflow sensor 5 and the rpm of theengine. The air pump 9 is driven from the time when the engine isstarted to the time when it is stopped, and keeps on introducing thesecondary air into the exhaust pipe 3.

As discussed above, the fresh air supplied through the secondary airintroduction system of FIG. 1 promotes the oxidation of noxiouscomponent such as HC and CO and thereby quickens the temperature rise ofthe catalytic converter. Thus, the time needed for the activation of thecatalyst is shortened (compared with the case where no secondary airintroduction system is provided) and the cleaning efficiency of thecatalytic converter rises relatively quickly.

However, the legal control against the noxious components contained inthe exhaust gas is becoming increasingly strict. The exhaust gasregulation of California is an example. Thus further reduction of thenoxious components in the exhaust gas is an urgent need.

FIG. 2 is a block diagram showing an internal combustion engine providedwith two catalytic converter units supplied with secondary air throughan air introduction pipe. The arrangement of FIG. 2 is disclosed, forexample, in Japanese Laid-Open Utility Model Application (Kokai) No.47-21018.

The exhaust gas cleaner system of FIG. 2 includes first and secondcatalytic converter units 7a and 7b. The second catalytic converter unit7b is disposed downstream of the first catalytic converter unit 7a. Thefirst and second catalytic converter units 7a and 7b accommodate thecatalytic converter rhodium, respectively. The air introduction pipe 8includes two branches for supplying separate amounts of secondary air tothe first and second catalytic converter units 7a and 7b. The amounts ofair supplied to the first and second catalytic converter units 7a and 7bare adjusted by a change-over valve 10a. In response to the output of atemperature sensor 21 disposed at the second catalytic converter unit7b, a controller 22 controls the change-over valve 10a and therebyadjusts the amounts of the secondary air supplied to the first andsecond catalytic converter units 7a and 7b. The parts not shown in FIG.2 are similar to those shown in FIG. 1.

The operation of the secondary air introduction system for the first andsecond catalytic converter units 7a and 7b of FIG. 2 is as follows. Whenthe choke valve is operated to restrict the airflow through the airintake pipe 2 at the start of the engine, the air/fuel mixture suppliedto the cylinders of the engine is rich in fuel content and the exhaustgas contains much HC and CO. Under this circumstance, the air sucked inby the air pump 9 from the air intake pipe 2 into the air introductionpipe 8 is divided by the change-over valve 10a into two portion suppliedthrough the two branches to the first and second catalytic converterunits 7a and 7b, respectively. The noxious components such as HC and COare thus oxidized into innocuous components such as H₂ O and CO₂ both inthe first and second catalytic converter units 7a and 7b, and areremoved from the exhaust gas.

When the temperature of the first and second catalytic converters 7a and7b rises and the catalysts are activated, the high temperature of thesecond catalytic converter unit 7b is detected by the temperature sensor21. In response to the output of the temperature sensor 21, thecontroller 22 changes over the change-over valve 10a to turn off thesupply of secondary air to the first catalytic converter unit 7a. Thesecondary air is thus supplied exclusively to the the second catalyticconverter unit 7b. The first catalytic converter 7a thus begins toefficiently reduce the nitrogen oxides NO_(x) contained in the exhaustgas to the nitrogen gas N₂. The remaining noxious components such as HCand CO are oxidized into innocuous components such as H₂ O and CO₂exclusively in the second catalytic converter unit 7b.

Even the exhaust gas cleaning system of FIG. 2 is not sufficientlyeffective. Namely, at the initial time when the first and the secondcatalytic converters are still at a lower temperature, the air lower intemperature than the exhaust gas are introduced into the exhaust pipe 3and mixed with the exhaust gas. The temperature of the exhaust gas isthus reduced, thereby slowing down the temperature rise of the catalyticconverters and delaying the full activation thereof. The initialcleaning efficiency is thus reduced. Furthermore, the air is distributedinitially in a fixed ratio to the first and the second catalyticconverters. The NO_(x), however, becomes increasingly abundant as thetemperature of the internal combustion engine 1 rises. Until thechange-over valve 10a is switched to turn off the air supply to thefirst catalytic converter unit 7a, this increasing amount of NO_(x) arenot removed efficiently.

FIG. 3 is a block diagram showing an internal combustion engine providedwith a catalytic converter supplied with heated secondary air through anair introduction pipe. In FIG. 3, the internal combustion engine 1 isshown with a transmission 1a and an AC generator 28. As in the case ofFIGS. 1 and 2, the air taken in through the air cleaner 4 is supplied tothe cylinders of the internal combustion engine 1 through the air intakepipe 2. The amount of airflow is controlled by the throttle valve 6 inthe air intake pipe 2. The exhaust gas is released to the atmospherethrough the exhaust pipe 3 provided with a catalytic converter unit 7.

The air sucked in by the electric air pump 9a into the air introductionpipe 8a is introduced into the exhaust pipe 3 through a flow controlvalve 10b, a check-valve 10, and a heater 23. The flow control valve 10bcontrols the amount of the air introduced into the exhaust pipe 3through the air introduction pipe 8a. The check-valve 10 prevents theexhaust gas from reversing through the air introduction pipe 8a. Theheater 23 heats the air before introducing it into the exhaust pipe 3.The controller unit 24 controls the operation of the flow control valve10b as well as the ON/OFF of the relays 26 and 27. The relays 26 and 27control the supply of current from the battery 25 to the electric airpump 9a and the heater 23, respectively.

The operation of the secondary air introduction system for the catalyticconverter unit 7 of FIG. 3 is as follows. As described above,immediately after the start of the engine, the air/fuel ratio is smalland the air/fuel mixture is rich in fuel content. The exhaust gas thuscontains large amounts of CO and HC. The catalytic converter is stillbelow the reaction (activation) temperature and is not sufficientlyactivated yet.

Simultaneously with, or after a predetermined length of time after, thestart of the engine, the controller unit 24 turns on the relay 26 forthe electric air pump 9a, thereby supplying power from the battery 25 tothe electric air pump 9a. The electric air pump 9a is thus driven andintroduces the secondary air into the exhaust pipe 3 through the airintroduction pipe 8a. Further, simultaneouly with, or after apredetermined length of time after, the start of the engine, thecontroller unit 24 turns on the relay 27 for the heater 23, therebysupplying power from the battery 25 to the heater 23. The heater 23 thusheats the secondary air before it is introduced into the exhaust pipe 3.

The heated secondary air introduced into the exhaust pipe 3 is suppliedto the catalytic converter unit 7 together with the exhaust gas, andaccelerates the temperature rise of the catalytic converter. Thecatalytic converter thus quickly reaches the reaction temperature and isactivated. The noxious components such as CO and HC are converted intothe innocuous components such as CO₂ and H₂ O in the catalytic converterunit 7. The exhaust gas passed through and cleaned by the catalyticconverter is released to the atmosphere.

During the above operation, the electric air pump 9a and the heater 23are operated simultaneously. The amount of consumed current is thuslarge, and the AC generator 28 keeps on charging the battery 25. Asshown in FIG. 4, the amount of secondary air introduced into the exhaustpipe 3 remains substantially constant. The amount of secondary air iscontrolled by the flow control valve 10b in the air introduction pipe8a.

The secondary air introduction system for the catalytic converter unit 7of FIG. 3 has the following disadvantage. During operation, the electricair pump 9a and the heater 23 is supplied with current from the battery25, which should thus be charged continually by the AC generator 28. Asa result, the AC generator 28 constitutes a heavy load on the internalcombustion engine 1. When the output of the internal combustion engine 1is reduced (as when the engine is idling), the concentration of thenoxious components in the exhaust gas increases due to the heavy loadimposed by the AC generator 28.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an exhaust gascleaner systems for cleaning the exhaust gas of an internal combustionengine which is capable of minimizing the amounts of the noxiouscomponents contained in the exhaust gas, especially during the timeimmediately after the engine is started.

The above object is accomplished in accordance with the principle ofthis invention by an exhaust gas cleaner system for an internalcombustion engine comprising: a catalytic converter mounted on anexhaust pipe of the internal combustion engine; an air introduction pipefor introducing secondary air into the exhaust pipe at a point upstreamof the catalytic converter; heater mounted on the air introduction pipefor heating the secondary air introduced into the exhaust pipe; andengine controller for controlling ignition timings of the internalcombustion engine, the engine controller retarding the ignition timingsat least during a part of an interval in which the secondary air heatedby the heater is introduced into the exhaust pipe.

Preferably, the engine controller retards the ignition timingintermittently. Where the internal combustion engine is a multi-cylinderinternal combustion engine, it is preferred that the engine controllercomprises: an ignition counter for counting a number of occurrences ofsuccessive ignitions of cylinders of the multi-cylinder internalcombustion engine, the ignition counter being reset to zero when thenumber of occurrences of successive ignitions reaches a predeterminednumber; wherein the engine controller retards ignition timings if andonly if the number of occurrences of successive ignitions counted andstored in the ignition counter falls within a predetermined range.

Further, it is preferred that the engine controller terminates retardingthe ignition timings before an introduction of the secondary air heatedby the heater is terminated.

In accordance with the second aspect of this invention, the above objectis accomplished by an exhaust gas cleaner system for an internalcombustion engine comprising: a first and a second catalytic convertermounted on an exhaust pipe of the internal combustion engine, the secondcatalytic converter being disposed to a downstream point of the firstcatalytic converter; an air introduction pipe for introducing secondaryair into the exhaust pipe, the air introduction pipe having a first anda second branch for introducing separate amounts of secondary air to thefirst and second catalytic converters, respectively; distributing meansfor distributing the separate amounts of the secondary air to the firstand second catalytic converters through the first and second branches ofthe air introduction pipe, respectively; first and second heatersmounted on the first and second branches of the air introduction pipefor heating independently the separate amounts of the secondary airdistributed to the first and second catalytic converters, respectively.

Preferably, the exhaust gas cleaner system comprises water temperaturedetector means for detecting a water temperature of the engine; and acontroller, coupled to an output of the water temperature detectormeans, for controlling the distributing means, wherein, when an outputof the water temperature detector means exceeds a first predeterminedlevel, the controller controls the distributing means such that a ratioof a percentage of the separate amount of the secondary air distributedto the first catalytic converter to a percentage of the separate amountof the secondary air distributed to the second catalytic converter isreduced to zero. Then, the controller may control the distributing meanssuch that: the ratio of the percentage of the separate amount of thesecondary air distributed to the first catalytic converter to thepercentage of the separate amount of the secondary air distributed tothe second catalytic converter is initially equal to a ratio ofcapacities of the first and second catalytic converters at a start ofthe internal combustion engine; and the ratio of the percentage of theseparate amount of the secondary air distributed to the first catalyticconverter to the percentage of the separate amount of the secondary airdistributed to the second catalytic converter is gradually reduced tozero as the output of the water temperature detector means rises to thefirst predetermined level.

Further, the controller may further control the operation of the firstheater, such that the first heater, turned on at a start of the internalcombustion engine, is turned off when the output of the watertemperature detector means exceeds a second predetermined level lowerthan the first predetermined level. Still further, the controller mayfurther control the operation of the second heater, such that the secondheater, turned on at a start of the internal combustion engine, isturned off when the output of the water temperature detector meansexceeds a third predetermined level higher than the first predeterminedlevel.

Alternatively, the exhaust gas cleaner system may comprise: a controllerfor controlling the distributing means, wherein the controller controlsthe distributing means such that a ratio of a percentage of the separateamount of the secondary air distributed to the first catalytic converterto a percentage of the separate amount of the secondary air distributedto the second catalytic converter is reduced to zero at an end of firstpredetermined length of time after a start of the internal combustionengine. Then, preferably, the controller controls the distributing meanssuch that: the ratio of the percentage of the separate amount of thesecondary air distributed to the first catalytic converter to thepercentage of the separate amount of the secondary air distributed tothe second catalytic converter is initially equal to a ratio ofcapacities of the first and second catalytic converters at a start ofthe internal combustion engine; and the ratio of the percentage of theseparate amount of the secondary air distributed to the first catalyticconverter to the percentage of the separate amount of the secondary airdistributed to the second catalytic converter is gradually reduced tozero toward the end of the first length of time.

Further, the controller may further control the operation of the firstheater, such that the first heater, turned on at a start of the internalcombustion engine, is turned off a second predetermined length of timebefore the end of the first predetermined length of time. Still further,the controller may further control the operation of the second heater,such that the second heater, turned on at a start of the internalcombustion engine, is turned off a third predetermined length of timeafter the end of the first predetermined length of time.

According to a third aspect of this invention, the above object isaccomplished by an exhaust gas cleaner system for an internal combustionengine comprising: a battery means; a generator means for charging thebattery means, the generator means being driven by the internalcombustion engine to generate electric power; a catalytic convertermounted on an exhaust pipe of the internal combustion engine; an airintroduction pipe for introducing secondary air into the exhaust pipe ata point upstream of the catalytic converter; an electric air pumpmounted on the air introduction pipe for forcing the secondary airtoward the exhaust pipe, the electric air pump being supplied with powerfrom the battery means; an electric heater mounted on the airintroduction pipe for heating the secondary air introduced into theexhaust pipe, the electric heater being supplied with power from thebattery means; and engine controller for controlling operation of thegenerator means, wherein the engine controller suspends generation ofpower performed by the generator means when at least either the electricair pump or the heater is being operated with power supplied from thebattery means.

Preferably, the engine controller suspends generation of power by thegenerator means only during the time when the internal combustion engineis within a predetermined length of time after a start. It is furtherpreferred that the engine controller further comprises detector meansfor detecting output voltage of the battery means and resumes thegeneration of power by the AC generator when the output voltage of thebattery means detected by the detector means falls below a predeterminedlevel.

BRIEF DESCRIPTION OF THE DRAWINGS

The features which are believed to be characteristic of this inventionare set forth with particularity in the appended claims. The structureand method of operation of this invention itself, however, will be bestunderstood from the following detailed description, taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a block diagram showing an internal combustion engine providedwith a catalytic converter supplied with secondary air through an airintroduction pipe;

FIG. 2 is a block diagram showing an internal combustion engine providedwith two catalytic converter units supplied with secondary air throughan air introduction pipe;

FIG. 3 is a block diagram showing an internal combustion engine providedwith a catalytic converter supplied with heated secondary air through anair introduction pipe;

FIG. 4 is a diagram showing the change of the secondary air flow ratefrom the start of the engine;

FIG. 5 is a block diagram showing an internal combustion engine providedwith a catalytic converter supplied with heated secondary air through anair introduction pipe according to this invention;

FIG. 6 is a timing chart showing the relationship among the rpm of theengine, the ON/OFF of the heater 23a, and the position of the flowcontrol valve 10b of FIG. 5;

FIG. 7 is a diagram showing the temperature rise curves of the catalystafter the start of the engine in the three cases where: the ignitiontiming is controlled normally and the secondary air is not heated (solidcurve A); the ignition timing is controlled normally while the secondaryair is heated (dot-and-dash curve B); and the ignition timing is delayedand the secondary air is heated (two-dots-and-dash curve C);

FIG. 8 is a timing chart showing the relationship among the output ofthe crank angle sensor 13, the ON/OFF of the igniter 16 and the ON/OFFof the ignition coil 15 of FIG. 5;

FIG. 9 is a flowchart showing the procedure by which the enginecontroller 14a determines the timings for turning on and off the igniter16 of FIG. 5;

FIG. 10 is a flowchart showing the procedure by which the enginecontroller 14a controls the ON/OFF of the igniter 16 of FIG. 5;

FIG. 11 compares the temperatures of the exhaust gas where the ignitiontiming is normal (the left column) and where the ignition timing isretarded (the right column);

FIG. 12 is a timing chart showing the relationships among variouswaveforms in the case where the ignition timings are retardedintermittently by means of a counter to improve the acceleratingperformance of the vehicle;

FIG. 13 is a flowchart showing the procedure by which the enginecontroller 14a sets or clears the secondary air introduction flag anddetermines the normal and retarded ignition timings;

FIG. 14 is a flowchart showing the procedure for determining theignition timings of the cylinders which are retarded intermittently;

FIG. 15 is a diagram showing the ignition timing control method by whichthe retardation of the ignition timing is terminated a predeterminedlength of time before the introduction of the secondary air isterminated;

FIG. 16 is a block diagram showing an internal combustion engineprovided with two catalytic converter units supplied with independentlycontrolled amounts of heated secondary air through an air introductionpipe according to this invention;

FIG. 17 is a timing chart showing the distribution ratio of thesecondary air and the operations of the first and second heaters 23b and23c in relation to the water temperature of the engine;

FIG. 18 is a timing chart showing the various waveforms occurring in thesystem of FIG. 16, in the case where the amount of secondary airintroduced into the exhaust pipe 3 is varied in accordance with theamount of the exhaust gas and the injector pulse width;

FIG. 19 is a timing chart showing the distribution ratio of thesecondary air and the operations of the first and second heaters 23b and23c in relation to the water temperature of the engine, in the casewhere the distribution ratio of the secondary air is varied gradually inresponse to the water temperature;

FIG. 20 is a timing chart showing the distribution ratio of thesecondary air and the operations of the first and second heaters 23b and23c, in the case where the distribution ratio of the secondary air isvaried gradually on the basis of the length of time from the start ofthe engine;

FIG. 21 is a block diagram showing an internal combustion engineprovided with a catalytic converter supplied with heated secondary airthrough an air introduction pipe, wherein the operation of the ACgenerator 28 (i.e., the generation of electric power) is suspended whilethe electric air pump 9a or the heater 23 is operated; and

FIG. 22 is a block diagram showing an internal combustion engineprovided with a catalytic converter supplied with heated secondary airthrough an air introduction pipe, wherein the operation of the ACgenerator 28 (i.e., the generation of electric power) is suspended whilethe electric air pump 9a or the heater 23 is operated, but is resumedwhen the battery voltage falls below a predetermined level.

In the drawings, like reference numerals represent like or correspondingparts or portions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the accompanying drawings, the preferred embodiments ofthis invention are described.

FIG. 5 is a block diagram showing an internal combustion engine providedwith a catalytic converter supplied with heated secondary air through anair introduction pipe according to this invention. The arrangement ofFIG. 5 is similar to that of FIG. 1, except for the following points.

In addition to the air pump 9 and the check-valve 10, the airintroduction pipe 8 is provided with a flow control valve 10b and aheater 23a. The flow control valve 10b controls whether or not thesecondary air is introduced into the exhaust pipe 3. The heater 23aheats the secondary air before it is introduced into the exhaust pipe 3.To the engine controller 14a are supplied the outputs of the airflowsensor 5, the air/fuel ratio sensor 11, and the crank angle sensor 13.In response thereto, the engine controller 14a controls the operationsof the flow control valve 10b and the heater 23a as well as theoperation of the fuel injectors 12. The details of the control isdescribed below. It is noted that the air pump 9 may be a mechanical airpump which produces an airflow corresponding to the rpm of the engine.Further, the secondary air may be introduced directly from theatmosphere into the air introduction pipe 8 (as in the case of thearrangement of FIG. 3).

The operation of the system of FIG. 5 is as follows. FIG. 6 is a timingchart showing the relationship among the rpm of the engine, the ON/OFFof the heater 23a, and the position of the flow control valve 10b ofFIG. 5. At the time point t₀, the rpm of the engine (the top waveform)reaches and exceeds a predetermined level S (e.g., 500 rpm), at which itis judged that the engine is started. Namely, it is judged that theengine is started when the rpm of the engine is above the predeterminedlevel S, and not started when the rpm of the engine is thereunder. Therpm of the engine is calculated from the output of the crank anglesensor 13. During a predetermined length of time T from the time pointt₀, the flow control valve 10b is opened (the bottom waveform), and theheater 23a is turned on (the middle waveform). At the time point t₁ atthe end of the interval T, the flow control valve 10b is closed and theheater 23a is turned off. The flow control valve 10b and the heater 23aare controlled by the engine controller 14a. Thus, during the timelength T after the engine is started, the secondary air heated by theheater 23a is introduced into the exhaust pipe 3, to accelerate thetemperature rise of the catalytic converter 7.

Further, as described in detail below by reference to FIGS. 8 through10, the ignition timings of the engine are retarded according to thisinvention. FIG. 7 is a diagram showing the temperature rise curves ofthe catalyst after the start of the engine in the three cases where: theignition timing is controlled normally and the secondary air is notheated (solid curve A); the ignition timing is controlled normally whilethe secondary air is heated (dot-and-dash curve B); and the ignitiontiming is delayed and the secondary air is heated (two-dots-and-dashcurve C). In FIG. 7, the activation temperature of the catalyst is shownat a representative level of 300 degrees. The comparison of the curves Aand B shows clearly that the heating of the secondary air is effectivein accelerating the temperature rise of the catalyst. As describedbelow, the curve C represents the temperature rise of the catalystaccording to this invention. The length of time T is selected longenough to encompass the interval between the time point t₀ at which theengine is started and the time point t₂ at which the curve C reaches andthen rises above the activation temperature.

FIG. 8 is a timing chart showing the relationship among the output ofthe crank angle sensor 13, the ON/OFF of the igniter 16 and the ON/OFFof the ignition coil 15 of FIG. 5. It is assumed that the engine is afour-cylinder four-cycle engine. Thus, the crank angle sensor 13 outputsa pulse corresponding to a cylinder for each 180 degrees of the crankangle (see the top waveform). The pulse rises at 75 degrees, and fallsat 5 degrees, before top dead center of the corresponding cylinder. Thepulse width corresponds to 70 degrees of the crank angle. The rpm of theengine is calculated from the pulse repetition period of the output ofthe crank angle sensor 13.

The engine controller 14a turns on the igniter 16 after a time lengthT_(IN) after the rising edge of a pulse of the output of the crank anglesensor 13 (i.e., at 75 degrees before top dead center of thecorresponding cylinder), and turns it off after a time length T_(SPARK)(see the middle waveform in FIG. 8). The current supply to the ignitioncoil 15 is turned on and off as the igniter 16 is turned on and off (seethe bottom waveform in FIG. 8). When the current supply to the ignitioncoil 15 is turned off, a high voltage is induced across the secondaryside of the ignition coil 15 to spark the ignition plug of theassociated cylinder. During the initial time immediately after the startof the engine, the time length T_(IN) is lengthened to retard theignition timing according to this invention, as described in detailbelow.

FIG. 9 is a flowchart showing the procedure by which the enginecontroller 14a determines the timings for turning on and off the igniter16 of FIG. 5. At step S1, the engine controller 14a determines the rpmof the engine on the basis of the output of the crank angle sensor 13.Further at step S2, the engine controller 14a reads in the output of theairflow sensor 5 and determines the amount of air taken into thecylinders of the engine. At step S3, the engine controller 14adetermines basic ignition angle (i.e., the crank angle before top deadcenter corresponding to the basic ignition timing). The basic ignitiontiming corresponds to the operating condition of the engine asdetermined from the rpm of the engine and the amount of air intake intothe cylinders. The basic ignition angles (the crank angles before topdead center corresponding to basic ignition timings) are stored in amemory (e.g., a ROM) of the engine controller 14a in the form of atable. By looking up the table entry corresponding to the operatingcondition of the engine as determined from the rpm of the engine and theamount of air intake into the cylinders, the engine controller 14adetermines the basic ignition angle.

At step S4, it is judged whether or not the flow control valve 10b isopen. If the judgement is negative at step S4 (i.e., if the flow controlvalve 10b is closed and hence the secondary air is not introduced intothe exhaust pipe 3), the execution proceeds to step S5 where theignition angle (the crank angle before top dead center corresponding tothe ignition timing) is set equal to the basic ignition angle determinedat step S3. On the other hand, if the judgement is affirmative at stepS4 (i.e., if the flow control valve 10b is open and hence the secondaryair is introduced into the exhaust pipe 3), the execution proceeds tostep S6, where the ignition angle (the crank angle before top deadcenter corresponding to the ignition timing) is retarded by apredetermined angle A from the basic ignition angle. Namely, theignition angle is set equal to:

(basic ignition angle)--A.

After step S5 or step S6, the execution proceeds to step S7, where theignition time is calculated from the ignition angle. Assuming that theengine is a four-cylinder four-cycle engine, the ignition time T_(IG) iscalculated by the formula:

    T.sub.IG sec.={(75°-I.sub.A °)/180 °}×30 sec./Ne

where I_(A) is the ignition angle (the crank angle before top deadcenter corresponding to the ignition timing) determined at step S5 orstep S6, and Ne is the rpm of the engine.

The ignition time T_(IG) calculated at step S7 represents the length oftime from the time point corresponding to 75 degrees before top deadcenter (i.e., the time point at the rising edge of a pulse of the outputof the crank angle sensor 13) to the ignition timing at which theigniter 16 is turned off to spark the ignition plug. Thus, as shown inFIG. 8, the ignition time T_(IG) calculated at step S7 is equal to thesum of: the time length T_(IN) (which extends from the time point at 75degrees before top dead center to the time point at which the igniter 16is turned on), and the time length T_(SPARK) (which extends from thetime point at which the igniter 16 is turned on to the time point atwhich the igniter 16 is turned off). The time length T_(SPARK) takes afixed predetermined value. Thus, at step S8, the time length T_(IN) iscalculated by

    T.sub.IN =T.sub.IG -T.sub.SPARK

FIG. 10 is a flowchart showing the procedure by which the enginecontroller 14a controls the ON/OFF of the igniter 16 of FIG. 5. At stepS9, the engine controller 14a determines whether or not the igniter 16is turned on. If the judgement is affirmative at step S9 (i.e., if theigniter 16 is turned on), the execution proceeds to step S11, where theigniter 16 is turned off at the end of the time length T_(SPARK) (seeFIG. 8). 0n the other hand, if the judgement is negative at step S9(i.e., if the igniter 16 is turned off), the execution proceeds to stepS10, where the igniter 16 is turned on after the time length T_(IN)after the time point at 75 degrees before top dead center. The interruptroutine of FIG. 10 is executed at predetermined timings (i.e., at eachtime point at 75 degrees before top dead center and at each time pointat which the igniter 16 is turned on).

FIG. 11 compares the temperatures of the exhaust gas where the ignitiontiming is normal (the left column) and where the ignition timing isretarded (the right column). (The normal ignition timing is the ignitiontiming determined from the basic ignition angle, and the retardedignition timing is the ignition timing determined from the ignitionangle which is retarded by the predetermined angle A from the basicignition angle.) As shown in FIG. 11, the temperature of the exhaust gasrises when the ignition timing is retarded. Thus the temperature of thecatalyst rises more quickly when the ignition timing is retarded. Thecomparison of the catalyst temperature curve B (representing the casewhere the ignition timing is controlled normally and the secondary airis heated) and the curve C (representing the case where the ignitiontiming is delayed and the secondary air is heated) in FIG. 7 shows thatthe retardation of the ignition timing according to this invention iseffective in shortening the time required to raise the catalyst abovethe activation temperature. Thus, the exhaust gas cleaning system ofFIG. 5 is effective in reducing the concentration of the noxiouscomponents in the exhaust gas.

In the case of the above described embodiment of FIG. 5, the ignitiontimings are retarded for all the cylinders. When, however, the ignitiontimings are retarded continuously for all the cylinders, the retardationof the ignition timings reduces the output torque of the engine. Thedrivability or the acceleration performance of the automobile is thusdeteriorated.

FIG. 12 is a timing chart showing the relationships among variouswaveforms in the case where the ignition timings are retardedintermittently by means of a counter to improve the acceleratingperformance of the vehicle. The details of the operations relating tothese waveforms will be described in detail below by reference to theflowcharts of FIGS. 13 and 14.

In FIG. 12, the top waveform represents the time points corresponding to75 degrees before top dead center before the ignition timings. At 75degrees before top dead center before each ignition timing, the ignitioncounter, provided in the engine controller 14a, is incremented (thebottom waveform). The secondary air introduction flag (the secondwaveform) is set when the flow control valve 10b is open, and is resetwhen the flow control valve 10b is closed. As shown by the solid curvein the third row in FIG. 12, the ignition angle is controlled normally(represented by β in FIG. 12) until the ignition counter exceeds Nn(Nn=3 in FIG. 12), and the ignition angle is retarded by an angle A (theretarded angle is represented by α in FIG. 12) when the ignition counteris from Nn up to, but not including, Nr (Nr=5 in FIG. 12), whereupon theignition counter is reset to 0. Thus three cylinders are ignitedsuccessively at the normal ignition timings and then two cylinders areignited successively at the retarded ignition timings. This cycle ofthree normal and two retarded ignition timing control is repeated untilthe secondary air introduction flag is cleared. The ignition counterserves as a modulo 5 counter, during the first three counts, 0, 1 and 2,of which the ignition timing is controlled normally, and during thesecond two counts, 3 and 4, of which the ignition timing is retarded.Thus, provided that the engine is a four-cylinder engine, the twocylinders whose ignition timings are retarded are successively changedas the cycle is repeated. By the way, the dotted curve in the third rowin FIG. 12 shows the case where the ignition timings of all thecylinders are retarded continually.

FIG. 13 is a flowchart showing the procedure by which the enginecontroller 14a sets or clears the secondary air introduction flag anddetermines the normal and retarded ignition timings. At step S1, theengine controller 14a determines the rpm of the engine on the basis ofthe output of the crank angle sensor 13. Further at step S2, the enginecontroller 14a reads in the output of the airflow sensor 5 anddetermines the amount of air taken into the cylinders of the engine. Atstep S3, the engine controller 14a determines basic ignition angle(i.e., the crank angle before top dead center corresponding to the basicignition timing). The basic ignition timing corresponds to the operatingcondition of the engine as determined from the rpm of the engine and theamount of air intake into the cylinders. The basic ignition angles (thecrank angles before top dead center corresponding to basic ignitiontimings) are stored in a memory (e.g., a ROM) of the engine controller14a in the form of a table. By looking up the table entry correspondingto the operating condition of the engine as determined from the rpm ofthe engine and the amount of air intake into the cylinders, the enginecontroller 14a determines the basic ignition angle.

At step S4, it is judged whether or not the flow control valve 10b isopen. If the judgement is negative at step S4 (i.e., if the flow controlvalve 10b is closed and hence the secondary air is not introduced intothe exhaust pipe 3), the execution proceeds to step S22 where theignition counter is cleared. Thereafter the execution proceeds to stepS5, where to the ignition angle β is assigned the basic ignition angledetermined at step S3.

On the other hand, if the judgement is affirmative at step S4 (i.e., ifthe flow control valve 10b is open and hence the secondary air isintroduced into the exhaust pipe 3), the execution proceeds to step S21,where the ignition counter is set. Thereafter the execution proceeds tostep S6, where the ignition angle α is assigned the value: (basicignition angle) --A.

FIG. 14 is a flowchart showing the procedure for determining theignition timings of the cylinders which are retarded intermittently. Theinterrupt routine of FIG. 14 is executed at each 75 degrees before topdead center and when the igniter 16 is turned on at the end of the timelength T_(IN) (see FIG. 8). When executed at 75 degrees before top deadcenter, the routine controls the commencement of the current supply tothe primary side of the ignition coil 15. On the other hand, whenexecuted at the time the igniter 16 is turned on, the routine controlsthe interruption of the current supply to the primary side of theignition coil 15.

At step S23, the engine controller 14a judges whether or not theinterrupt occurred at 75 degrees before top dead center. If thejudgement is negative at step S23, the execution proceeds to step S33,where the time length T_(IN) from the time point corresponding to 75degrees before top dead center to the ignition timing is calculated.When the execution proceeds to step S33, the ignition time T_(IG) isalready calculated during the previous execution cycle. Further, timelength T_(SPARK) during which the current is supplied to the ignitioncoil 15 is fixed. Thus, the time length T_(IN) is calculated by:

    T.sub.IN =T.sub.IG -T.sub.SPARK

After step S33, the routine similar to that of FIG. 10 is performed.

On the other hand, if the judgement is affirmative at step S23 (i.e., ifthe interrupt occurred at 75 degrees before top dead center), theexecution proceeds to step S24, where it is judged whether or not thesecondary air introduction flag is set. If the judgement is negative atstep S24 (i.e., if the secondary air introduction flag is not set andhence the heated secondary air is not being introduced into the exhaustpipe 3), the execution proceeds to step S25, where the ignition counteris reset to 0. Thereafter, at step S26, the ignition time T_(IG) (whichrepresents the length of time from the time point corresponding to 75degrees before top dead center to the ignition timing) is calculatedfrom the ignition angle β:

    T.sub.IG sec.={ (75°-β°)/180°}×30 sec./Ne

where Ne is the rpm of the engine.

After step S26, the execution proceeds to step S33.

On the other hand, if the judgement is affirmative at step S24 (i.e., ifthe secondary air introduction flag is set and hence the heatedsecondary air is being introduced into the exhaust pipe 3), theexecution proceeds to step S27, where it is judged whether or not theignition counter is greater than the first predetermined number Nn. Ifthe judgement is negative at step S27, the execution proceeds to stepS28, where the ignition time T_(IG) is calculated from the ignitionangle β. On the other hand, if the judgement is affirmative at step S27,the execution proceeds to step S30, where it is judged whether or notthe ignition counter is greater than the second predetermined number Nr.If the judgement is negative at step S30, the ignition time T_(IG) iscalculated from the ignition angle α:

    T.sub.IG sec.={ (75°-α°)/180°}×30 sec./Ne

where Ne is the rpm of the engine.

If the judgement is affirmative at step S30 (i.e., if the ignitioncounter is greater than the second predetermined number Nr), theexecution proceeds to step S31, where the ignition counter is reset to0. Thereafter, the execution proceeds to step S28, where the ignitiontime T_(IG) is calculated from the ignition angle β.

After step S28 or step S32, the execution proceeds to step S29, wherethe ignition counter is incremented. Thereafter, at step S33, the timelength Thd IN is calculated from the ignition time T_(IG).

After step S33, the engine controller 14a controls the igniter 16 inaccordance with the procedure of FIG. 10.

In accordance with the control methods shown in FIGS. 13 and 14, theignition timings are retarded intermittently until the introduction ofthe heated secondary air is terminated. The intermittent retardation ofthe ignition timing is effective in improving the drivability(accelerating performance) of the vehicle. Another method of improvingthe accelerating performance of the vehicle is terminating theretardation of the ignition timing a predetermined length of time beforethe introduction of the secondary air is terminated.

FIG. 15 is a diagram showing the ignition timing control method by whichthe retardation of the ignition timing is terminated a predeterminedlength of time before the introduction of the secondary air isterminated. Before the introduction of the secondary air into theexhaust pipe 3 is stopped at the time point t₄ at the end of theinterval T (see the middle waveform in FIG. 15), the ignition timing isreturned to the normal timing at time point t₃ (see the bottomwaveform). The length of time T_(s) during which the ignition timing isretarded is shorter than the length of time T during which the secondaryair is introduced into the exhaust pipe 3. The solid curve D at the topof FIG. 15 represents the catalyst temperature rise in the case wherethe ignition timing is retarded until the time point t₄ at which theintroduction of the secondary air is terminated. The dot-and-dash curveE represents the catalyst temperature rise in the case where theretardation of the ignition timing is terminated at the time point t₃.The length of time T_(s) is selected such that the difference betweenthe two temperature rise curves D and E is sufficiently small. It isnoted that, as shown at the top in FIG. 15, a substantial margin isprovided between the time point t₂ at which the curve D crosses thecatalyst activation temperature of 300 degrees and the time point t₄ atwhich the introduction of the secondary air is terminated. The controlmethod of the ignition timing in accordance with FIG. 15 is effective inimproving the drivability (acceleration performance) of the vehicle.

FIG. 16 is a block diagram showing an internal combustion engineprovided with two catalytic converter units supplied with independentlycontrolled amounts of heated secondary air through an air introductionpipe according to this invention. The arrangement, of FIG. 16 is similarto that of FIG. 2, except where stated otherwise.

In FIG. 16, the fuel injectors 12 are provided for respective cylindersof the engine in the respective branches of the air intake pipe 2. Thefuel injectors 12 are controlled by the engine controller 14b. Anairflow sensor 5 disposed on the air intake pipe 2 upstream of thethrottle valve 6 detects the rate of airflow taken into the cylinders ofthe internal combustion engine 1. An air/fuel ratio sensor 11 disposedon the exhaust pipe 3 detects the air/fuel ratio supplied to thecylinders from the oxygen concentration contained in the exhaust gas. Awater temperature detector 30 detects the water temperature (thetemperature of the water in the jacket surrounding the cylinders of theengine). The output signals of the airflow sensor 5, the air/fuel ratiosensor 11, and the water temperature detector 30 are supplied to theengine controller 14b.

The secondary air taken into the air introduction pipe 8 by means of theair pump 9 is distributed by the change-over valve 10a to the first andsecond branches 8b and 8c. The change-over valve 10a is capable ofdistributing the secondary air in a variable proportion to the twobranches. The first branch 8b is provided a first check-valve 10c and afirst heater 23b. The second branch 8c is provided a second check-valve10d and a second heater 23c. The first heater 23b heats the secondaryair introduced into the exhaust pipe 3 upstream of the first catalyticconverter unit 7a. The first check-valve 10c prevents the reverse flowthrough the first branch 8b. The second heater 23c heats the secondaryair introduced into the exhaust pipe 3 upstream of the second catalyticconverter unit 7b. The second check-valve 10d prevents the reverse flowthrough the second branch 8c. The air pump 9, the change-over valve 10a,the first heater 23b, the second heater 23c are controlled by the enginecontroller 14b. The air pump 9 is preferably an electric air pump.

The engine controller 14b determines the basic fuel injection pulsewidth on the basis of the amount of air intake (the airflow rate)detected by the airflow sensor 5 and the rpm of the engine. Further, theengine controller 14b effects the temperature correction with respect tothe basic fuel injection pulse width on the basis of water temperaturein the jacket surrounding the cylinders of the engine. Further,performing the feedback correction of the air/fuel ratio on the basis ofthe output of the air/fuel ratio sensor 11 to adjust the air/fuel ratioto the thoretical air/fuel ratio, the engine controller 14b determinesthe injection pulse width and drives the fuel injectors 12 and controlsthe amount of injected fuel by means of the fuel injection signals.

Furthermore, the engine controller 14b controls the rpm of the air pump9 to adjust the amount of secondary air taken into the air introductionpipe 8. Still further, in response to the temperature of the water inthe jacket of the engine, the engine controller 14b controls thechange-over valve 10a to adjust the ratio of the amounts of thesecondary air distributed to the first and second catalytic converterunits 7a and 7b, respectively. Still further, the engine controller 14bcontrols the current supply to the first and second heaters 26 and 27(i.e., turns them on and off) to adjust the temperatures of thesecondary air supplied to the first and second catalytic converter units7a and 7b.

Next, the operation of the arrangement of FIG. 16 is described.

When the internal combustion engine 1 is still in the cold stateimmediately after it is started, the amount of the injected fuel iscontrolled by the engine controller 14b by the open loop control methodto produce the air/fuel mixture richer than the theoretical air/fuelratio. Thus, the exhaust gas contains abundance of noxious components,HC and CO, which are to be oxidized for removal. The first and secondcatalytic converters 7a and 7b, however, are both still not activatedimmediately after the engine is started. Thus, immediately after thestart of the engine, the engine controller 14b operates the air pump 9and controls the change-over valve 10a such that the secondary air isintroduced to both the first and second catalytic converters 7a and 7b.The secondary air is heated by the heaters 23b and 23c, respectively,before introduction into the exhaust pipe 3. This prevents thetemperature fall of the exhaust gas by the secondary air.

The exhaust gas is maintained at a high temperature and the oxidationreaction proceeds at the first and second catalytic converters 7a and7b, such that the activation of the first and second catalyticconverters 7a and 7b is promoted. As a result, the noxious componentsgenerated immediately after the start of the engine, HC and CO, areoxidized efficiently into CO₂ and H₂ O by the oxygen contained in thesecondary air. While the engine is in the cold state immediately afterthe start of the engine, the nitrogen oxides NO_(x) are hardlygenerated. Thus, the secondary air is introduced not only to the secondcatalytic converter unit 7b but also to the first catalytic converterunit 7a, such that HC and CO are cleaned efficiently in both thecatalytic converter units.

FIG. 17 is a timing chart showing the distribution ratio of thesecondary air and the operations of the first and second heaters 23b and23c in relation to the water temperature of the engine. The watertemperature (the top curve) detected by the water temperature detector30 rises gradually after the engine is started. Based on the watertemperature detected by the water temperature detector 30, the enginecontroller 14b infers the temperature of the first catalytic converterunit 7a. When it is judged that the first catalytic converter 7a isactivated (at the time point t₁₀ in FIG. 17), the engine controller 14bcontrols the change-over valve 10a such that the secondary air isintroduced only to the second catalytic converter unit 7b. Thus, thepercentage of the secondary air distributed to the first catalyticconverter unit 7a (curve R₁) falls to 0 percent at the time point t₁₀,while the ratio of the secondary air distributed to the second catalyticconverter unit 7b (curve R₂ ) rises to 100 percent at the same timepoint t₁₀. Before the time point t₁₀, the secondary air is distributedin a predetermined fixed ratio to the first and second catalyticconverter units 7a and 7b. At the time point t₁₀, the first heater 23bis turned off (the curve H₁ in FIG. 17).

The second catalytic converter unit 7b, however, is situated downstreamof the first catalytic converter unit 7a. As a result, the temperatureof the exhaust gas introduced into the second catalytic converter unit7b is lower than that of the exhaust gas introduced into the firstcatalytic converter unit 7a, and the activation of the second catalyticconverter is delayed than that of the first catalytic converter. Thus,the second heater 23c is kept turned on (the curve H₂ in FIG. 17) toheat the secondary air introduced to the second catalytic converter unit7b.

After the time point t₁₀, HC and CO are removed at the second catalyticconverter unit 7b by means of the secondary air supplied thereto. Thenitrogen oxides NO_(x), the amounts of which increase as the enginewarms up, are reduced to innocuous N₂ at the first catalytic converterunit 7a.

When the engine is further warmed and it is judged that the secondcatalytic converter is also activated, the engine controller 14b turnsoff the second heater 23c at the time point t₁₁ (see the curve H₂ inFIG. 17). After the time point t₁₁, the unheated secondary air at theroom temperature is introduced to the second catalytic converter unit7b. Thus, the nitrogen oxides NO_(x) and a portion of HC and CO areremoved by the oxidation and reduction reaction at the first catalyticconverter unit 7a, and the remaining HC and CO are removed by theoxidation reaction at the second catalytic converter unit 7b. Theheating of the secondary air introduced to the first and secondcatalytic converter units 7a and 7b immediately after the start of theengine promotes the activation of the first and second catalyticconverters 7a and 7b and improves the cleaning efficiency of HC and CO.

In the case of the conventional system, the air pump is driven directlyby the engine through a belt, etc., such that the amount of secondaryair introduced into the exhaust pipe 3 may become inappropriate. In thecase of the above embodiment of FIG. 16, however, the air pump 9 iscontrolled independently of the engine, such that the amount ofsecondary air introduced to the first and second catalytic converters 7aand 7b can be controlled appropriately and the cleaning efficiency isfurther improved. Although, in accordance with the method of operationshown in FIG. 17, a fixed amount of secondary air is introduced into theexhaust pipe 3, the amount of HC and CO contained in the exhaust gaschanges in accordance with the operating condition of the engine, asdescribed below.

First, the amount of exhaust gas is proportional to the amount of airtaken into the cylinder. Thus, provided that the concentrations of HCand CO remain constant, the (absolute) amount of HC and CO increaseswhen the amount of air intake into the cylinders increases. Thus,detecting the amount of air intake from the output of the airflow sensor5, the engine controller 14b may adjust the output level air pump 9.

Second, when the injector pulse width is varied, the air/fuel ratiosupplied to the cylinders and hence the concentrations of HC and COcontained in the exhaust gas change. Thus, the engine controller 14b maycontrol the output level of the air pump 9 to adjust the amount ofsecondary air in response to the concentration of HC and CO in theexhaust gas. The injector pulse width and hence the concentration of HCand CO in the exhaust gas may vary due to the ON/OFF of the feedbackcontrol of the air/fuel ratio, and, during the feedback control of theair/fuel ratio, due to the variation of the output of the enginecontroller 14b.

FIG. 18 is a timing chart showing the various waveforms occurring in thesystem of FIG. 16, in the case where the amount of secondary airintroduced into the exhaust pipe 3 is varied in accordance with theamount of the exhaust gas and the injector pulse width. At the timepoint t₂₁, the vehicle begins to be accelerated and the amount of airtaken into the cylinders of the internal combustion engine 1 increasesfrom the time point t₂₁ to the time point t₂₂, as shown by the curve at(a). The amount of exhaust gas (d) increases accordingly from the timepoint t₂₁ to the time point t₂₂. During the interval from the time pointt₂₁ to the time point t₂₂, the engine controller 14b increases theamount of secondary air (f) in proportion to the amount of exhaust gas(d). During the same interval from the time point t₂₁ to the time pointt₂₂, the driving pulse width (b) of the fuel injectors 12(.corresponding to the amount of fuel) is increased accordingly, suchthat the air/fuel ratio (c) and the concentration of HC and CO (e)remain substantially constant. During the interval from the time pointt₂₂ to the time point t₂₃, the engine is in a stable state, and all thecurves (a) through (f) are maintained substantially at the constantlevel. In particular, the air/fuel ratio (c) is maintained at thetheoretical air/fuel ratio of 14.7. The pulse width of the fuelinjectors 12 corresponding to the theoretical air/fuel ratio during thisinterval is taken as the standard or reference level for the subsequentcontrol, and is designated as 100 percent level.

After the time point t₂₃, the injector pulse width (b) is varied fromthe reference level of 100 percent. Namely, from the time point t₂₃ tothe time point t₂₄, the injector pulse width (b) is increased; from thetime point t₂₄ to the time point t₂₅ the injector pulse width isdecreased; and from the time point t₂₅ to the time point t₂₆, theinjector pulse width is increased again. The air/fuel ratio (c) rises(becomes lean) when the injector pulse width falls, and falls (becomesrich) when the injector pulse width rises. Accordingly, theconcentration of HC and CO (e) rises and falls as the injector pulsewidth increases and decreases. Thus, for supplying the appropriateamount of oxygen needed to oxidize HC and CO contained in the exhaustgas, the amount of secondary air (f) is increased and decreasedproportionally as the injector pulse width is increased and decreased.

In the control method shown in FIG. 17, the secondary air is distributedin a fixed ratio to the first and second catalytic converter units 7aand 7b. FIG. 19 is a timing chart showing the distribution ratio of thesecondary air and the operations of the first and second heaters 23b and23c in relation to the water temperature of the engine, in the casewhere the distribution ratio of the secondary air is varied gradually inresponse to the water temperature. At the time point t₃₀ immediatelyafter the start of the engine, the ratio of the percentage R₁ of thesecondary air distributed to the first catalytic converter unit 7a tothe percentage R₂ of the secondary air distributed to the secondcatalytic converter unit 7b is set equal to the ratio of the capacitiesof the first and second catalytic converter units 7a and 7b. Thereafter,as the water temperature rises, the percentage R₁ of the secondary airdistributed to the first catalytic converter unit 7a is decreasedgradually such that the amount of secondary air introduced to the firstcatalytic converter unit 7a vanishes when, at the time point t₃₂, thewater temperature reaches the first predetermined level at which it isjudged that the first catalytic converter has reached the activationtemperature. Thus, the percentage R₂ of the secondary air distributed tothe second catalytic converter unit 7b is increased gradually andreaches 100 percent at the time point t₃₂.

The first heater 23b is turned off when the water temperature of theengine rises to a second level (curve H₁). The engine controller 14bturns off the second heater 23c at the time point t₃₃ when the engine isfurther warmed and it is judged that the second catalytic converter isalso activated (curve H₂). The engine controller 14b may turn off thesecond heater 23c when the output of the water temperature detector 30reaches a predetermined third level higher than the first levelcorresponding to the time point t₃₂. Alternatively, the enginecontroller 14b may turn off the second heater 23c after a predeterminedlength of time after the time point t₃₂.

The amount of nitrogen oxides NO_(x) increases as the temperature of theinternal combustion engine 1 rises. In accordance with the controlmethod of the change-over valve 10a as represented by the curves R₁ andR₂ in FIG. 19, the cleaning efficiency of the nitrogen oxides NO_(x) bythe first catalytic converter unit 7a is increased in accordance withthe increasing amount of NO_(x). The amount of NO_(x) contained in theexhaust gas can thus be limited under a predetermined level.

FIG. 20 is a timing chart showing the distribution ratio of thesecondary air and the operations of the first and second heaters 23b and23c, in the case where the distribution ratio of the secondary air isvaried gradually on the basis of the length of time from the start ofthe engine. The ratio of the percentage R₁ of the secondary airdistributed to the first catalytic converter unit 7a to the percentageR₂ of the secondary air distributed to the second catalytic converterunit 7b is set equal to the ratio of the capacities of the first andsecond catalytic converter units 7a and 7b at the time point t₄₀ atwhich the engine is started. Thereafter, the percentage R₁ of thesecondary air distributed to the first catalytic converter unit 7a isdecreased linearly to zero over a predetermined length of time ending atthe time point t₄₂. The first heater 23b is turned off at the time pointt₄₁ a predetermined length of time before the time point t₄₂. On theother hand, the second heater 23c is turned off at the time point t₄₃ apredetermined length of time after the time point t₄₂.

As shown in FIGS. 19 and 20, the first heater 23b is turned off at thetime point t₃₁ (FIG. 19) or the time point t₄₁ (FIG. 20) before thepercentage R₁ of the secondary air distributed to the first catalyticconverter unit 7a is reduced to zero. In FIG. 19, the first heater 23bis turned off at the time point t₃₁ when the water temperature reaches apredetermined level. In FIG. 20, the first heater 23b is turned off atthe time point t₄₁ when a predetermined length of time from the start ofthe engine (at the time point t₄₀) is terminated (at the time pointt₄₁). The over-heating of the first heater 23b which may result from thedecreasing amount of air supplied thereto can thus be prevented. Thefailure of the first heater 23b such as the disconnection of the wirecan thus be avoided. The heat remaining in the first heater 23b after itis turned off is sufficient for heating the secondary air introduced tothe first catalytic converter unit 7a. Thus the exhaust gas isintroduced thereto is not cooled by the secondary air. The difference inthe water temperatures corresponding to the time point t₃₁ and timepoint t₃₂ in FIG. 19, or the length of time between the time point t₄₁and time point t₄₂ in FIG. 20, is selected to ensure that the heatremaining after the first heater 23b is turned off is sufficient forheating the secondary air introduced to the first catalytic converterunit 7a.

FIG. 21 is a block diagram showing an internal combustion engineprovided with a catalytic converter supplied with heated secondary airthrough an air introduction pipe, wherein the operation of the ACgenerator 28 (i.e., the generation of electric power) is suspended whilethe electric air pump 9a or the heater 23 is operated. The arrangementof FIG. 21 is similar to that of FIG. 3, except where stated otherwise.

The controller unit 24a controls the position of the flow control valve10b and the ON/OFF of the relays 26 and 27. Further through a signalline 28a, the controller unit 24a controls the operation (i.e., thegeneration of electric power) of the AC generator 28.

Next the operation of the system of FIG. 21 is described. Simultaneouslywith, or after a predetermined length of time after, the start of theengine, the controller unit 24a turns on the relay 26 for the electricair pump 9a, thereby supplying power from the battery 25 to the electricair pump 9a. The electric air pump 9a is thus driven and introduces thesecondary air into the exhaust pipe 3 through the air introduction pipe8a. Further, simultaneouly with, or after a predetermined length of timeafter, the start of the engine, the controller unit 24a turns on therelay 27 for the heater 23 (which, for example, may be a resistanceheater), thereby supplying power from the battery 25 to the heater 23.The heater 23 thus heats the secondary air before it is introduced intothe exhaust pipe 3. The amount of secondary air introduced into theexhaust pipe 3 is controlled by means of the flow control valve 10b. Thesecondary air is preferably heated to a temperature (usually from 300 to400 degrees centigrade) that is higher than the temperature of theexhaust gas produced immediately after the start of the engine.

The secondary air introduced into the exhaust pipe 3 is supplied to thecatalytic converter unit 7 together with the exhaust gas. Thetemperature rise of the catalytic converter 7 is thus accelerated by theheated secondary air supplied thereto, and the catalyst reaches quicklyto the activation or reaction temperature thereof. The noxiouscomponents CO and HC contained in the exhaust gas is converted intoinnocuous components CO₂ and H₂ O at the catalytic converter unit 7 andthe cleaned exhaust gas is released to the atmosphere.

The introduction of the secondary air heated by the heater 23 and drivenby the electric air pump 9a usually continues until the catalyticconverter is activated and the exhaust gas is stably cleaned. Thisusually extends over several minutes.

When either the electric air pump 9a or the heater 23 is being operated,the controller unit 24a outputs via a signal line 28a a power generationstop command to the AC generator 28 and stops the power generation ofthe AC generator 28. On the other hand, when neither the electric airpump 9a nor the heater 23 is being operated, the controller unit 24aoutputs via a signal line 28a a power generation command to the ACgenerator 28 and operates the AC generator 28 to generate power. Thebattery 25 is charged during the time the AC generator 28 generatespower.

Thus, the power generation operation of the AC generator 28 is stoppedwhen the electric air pump 9a or the heater 23 is in operation. Theheavy load imposed by the AC generator 28 on the internal combustionengine 1 if the AC generator 28 is operated when the electric air pump9a or the heater 23 is in operation can thus be eliminated, therebysuppressing the increase of the amount of the noxious components,especially under low output power condition of the engine. Further, theoperation of the engine under low output power condition (e.g., when theengine is idling) is rendered more stable. On the other hand, thebattery 25 can be charged sufficiently while the electric air pump 9aand the heater 23 are not operated.

In the case of the above embodiment of FIG. 21, the generation of powerby the AC generator 28 is stopped when either the electric air pump 9aor the heater 23 is operated. This suspension of the generation of powerby the AC generator 28 may only be performed during a predeterminedlength of time after the start of the engine. Namely, for apredetermined length of time, e.g., for about several tens of seconds,after the start of the engine, the temperature of the engine is stilllow and the circulation system thereof is not in good order. Thus, therotation of the engine is still unstable. In a preferred embodiment,only during this initial low temperature period in which the rotation ofthe engine is unstable, the generation of power by the AC generator 28is stopped when either the electric air pump 9a or the heater 23 isoperated. By limiting the suspension of the generation of power within abrief initial period, the chance of over-discharging the battery 25 caneffectively eliminated.

FIG. 22 is a block diagram showing an internal combustion engineprovided with a catalytic converter supplied with heated secondary airthrough an air introduction pipe, wherein the operation of the ACgenerator 28 (i.e., the generation of electric power) is suspended whilethe electric air pump 9a or the heater 23 is operated, but is resumedwhen the battery voltage falls below a predetermined level. In additionto the parts shown in FIG. 21, the arrangement of FIG. 22 includes avoltage detector line 25a for detecting the voltage of the battery 25.As in the case of the embodiment of FIG. 21, the operation of the ACgenerator 28 is suspended when either the electric air pump 9a or theheater 23 is operated. The controller unit 24a, however, continuallymonitors the terminal voltage of the battery 25 via the voltage detectorline 25a. When the terminal voltage of the battery 25 falls below apredetermined level, the controller unit 24a outputs via the signal line28a a power generation command to the AC generator 28 and resumes thegeneration of power by the AC generator 28. Thus, the over-discharge ofthe battery 25 can be effectively prevented, and the reduction in theperformance of the electric air pump 9a and the heater 23 resulting fromthe low voltage level of the battery 25 can be prevented.

What is claimed is:
 1. An exhaust gas cleaner system for an internalcombustion engine comprising:a catalytic converter mounted on an exhaustpipe of said internal combustion engine; an air introduction pipe forintroducing secondary air into said exhaust pipe at a point upstream ofsaid catalytic converter; heater mounted on said air introduction pipefor heating said secondary air introduced into said exhaust pipe; andengine controller for controlling ignition timings of said internalcombustion engine, said engine controller retarding said ignitiontimings at least during a part of an interval in which said secondaryair heated by said heater is introduced into said exhaust pipe; whereinsaid engine controller terminates retarding said ignition timings beforean introduction of said secondary air heated by said heater isterminated.
 2. An exhaust gas cleaner system for an internal combustionengine as claimed in claim 1, wherein said engine controller retardssaid ignition timing intermittently.
 3. An exhaust gas cleaner systemfor an internal combustion engine, comprising:a catalytic convertermounted on an exhaust pipe of said internal combustion engine; an airintroduction pipe for introducing secondary air into said exhaust pipeat a point upstream of said catalytic converter; heater mounted on saidair introduction pipe for heating said secondary air introduced intosaid exhaust pipe; and engine controller for controlling ignitiontimings of said internal combustion engine, said engine controllerretarding said ignition timings at least during a part of an interval inwhich said secondary air heated by said heater is introduced into saidexhaust pipe; wherein said engine controller retards said ignitiontiming intermittently; wherein said internal combustion engine is amulti-cylinder internal combustion engine and said engine controllercomprises an ignition counter for counting a number of occurrences ofsuccessive ignitions of cylinders of said multi-cylinder internalcombustion engine, said ignition counter being reset to zero when saidnumber of occurrences of successive ignitions reaches a predeterminednumber; and wherein said engine controller retards ignition timings ifand only if said number of occurrences of successive ignitions countedand stored in said ignition counter falls within a predetermined range.4. An exhaust gas cleaner system for an internal combustion engine asclaimed in claim 3, wherein said engine controller terminates retardingsaid ignition timings before an introduction of said secondary airheated by said heater is terminated.